Sol gel, and particularly seeded sol gel abrasives, have demonstrated substantial advantages over other premium abrasives in broad areas of bonded abrasive applications since their introduction some few years ago. Such abrasives are generally made by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2. The dried material is crushed either before or after sintering to obtain irregular blocky shaped polycrystalline abrasive grits in a desired size range. The grits may later be incorporated in a bonded abrasive product such as a grinding wheel or a segment.
U.S. Pat. No. 4,314,827 to Leitheiser et al. discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns in diameter. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases such as a finely crystalline alumina magnesia spinel.
U.S. Pat. No. 4,623,364, which issued on Nov. 18, 1986 assigned to Norton Company, the assignee of this application, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibratory milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion, the rods formed are later cut or broken into appropriate lengths.
U.S. Pat. No. 4,744,802, which issued May 17, 1988, also discloses a seeded sol gel process for producing alpha alumina based ceramics useful as abrasive grain and ceramic shaped bodies. Such alpha alumina is obtained from alpha alumina monohydrate to which has been added a nucleating agent.
Once the gel has formed, it may be shaped, according to the patentee, by any convenient method such as pressing, molding or extrusion and then carefully dried to produce an uncracked body of the desired shape. If abrasive material is desired, the gel can be extruded, according to the disclosure, or simply spread out to any convenient shape and dried. After drying, the solid body or material can be cut or machined to form a desired shape or crushed or broken by suitable means, such as a hammer or ball mill, to form abrasive particles or grains.
Such seeded sol gel abrasives have a much firmer alpha Al.sub.2 O.sub.3 crystal structure and higher density than the Leitheiser-type unseeded sol gel material. The alpha Al.sub.2 O.sub.3 crystals of the seeded sol gel abrasives are submicron and usually on the order of about 0.4 microns and less, although somewhat coarser structure may result if the seeding is performed in a non-optimal manner or if the firing is at too high a temperature, or for too long a duration.
Other materials such as iron oxide, chromium oxide, gamma alumina, and precursors of these oxides, as well as other fine debris that will act as nucleating sites for the alpha alumina crystals being formed, can also be used as seeds to facilitate the conversion to alpha Al.sub.2 O.sub.3. As a rule of thumb, such seeding materials should be isostructural with Al.sub.2 O.sub.3 and should have similar (within about 15%) crystal lattice parameters to work well.
U.S. Pat. Nos. 3,183,071 to Rue et al. and 3,481,723 to Kistler et al. disclose grinding wheels for use in heavy duty snagging operations made with extruded rod shaped polycrystalline alpha alumina abrasive grits. Kistler et al. refers broadly to the use of extruded polycrystalline sintered alumina abrasive rods with diameters of the order of about 26 to 160 mils (0.65 to 3.28 mm) which are formed by extruding a slurry of alpha Al.sub.2 O.sub.3 or other suitable fine ceramic particles which have been mixed with organic binding agents to facilitate the extrusions.
Similarly, Howard in U.S. Pat. No. 3,387,957 of June 11, 1968 extrudes bauxite as small diameter straight cylindrical rods to lengths longer than the diameter for use as abrasive in resin-bonded snagging wheels.
The rod shaped abrasive grits of the Rue '071, Kistler '723, and Howard '957, are intended for heavy duty snagging operations on steel and then the rod shaped abrasive grits are in practice rather coarse, generally a rod diameter equivalent to a size 16 grit or coarser. While it is possible, in theory, to make finer grit having smaller cross sections and diameters, it would be necessary to incorporate excessive amounts of organic binders, extrusion aids, and lubricants in the slurry in order to be able to extrude it through the finer holes. These additives would all have to be burnt out during sintering which would result in either excessive porosity and therefore weakness in the sintered rods or would require excessive firing in order to densify the material after the additives are burned out. The high firing would result in excessive and undesirable grain growth in the product.